It is now clear that B cells can play an important role downregulating immune responses in an IL-10-dependent fashion. Other than a capacity to express IL-10 after ex vivo stimulation, these rare regulatory B cells (Breg) lack specific markers and are found within various B cell subpopulations where their frequency can be as low as 1%. This has made study of Breg challenging. Surprisingly, a number of agents believed to induce allograft tolerance through their effects on T cells, were recently shown to be B cell dependent. This suggests that Breg may be involved, and play a broad role in allograft tolerance. This is echoed by the B cell profile observed in tolerant human renal allograft recipients. T cell Ig Mucin protein-1 helps regulate Th differentiation. Anti- TIM-1 (RMT1-10) induces allograft tolerance thought to be Th2 dependent. Surprisingly, we found TIM-1 is expressed at much higher levels on B than T cells. Moreover, Th2 deviation and allograft tolerance induced by anti-TIM-1, were completely B cell dependent. In fact, in the absence of B cells, anti-TIM-1 actually accelerates rejection compared to untreated controls. TIM-1 is expressed by the large majority of IL-10+ and IL-4+ B cells in all major subpopulations. TIM-1+, but not TIM-1-, B cells from allograft recipients can transfer tolerance to otherwise untreated allograft recipients. Finally, anti-TIM-1 significantly induces IL-10+ (TIM-1+) Breg, but both Breg development and induction require IL-4 signaling. Armed with these novel insights, we can now establish the role of TIM-1+ Breg allograft tolerance and determine the role of TIM-1 in Breg generation. Specifically, In Aim 1, we will define the role of TIM-1+ Breg in allograft tolerance using TIM-1 loss of function (LOF) KI mice, and determine whether TIM-1+ B cells are the specific B cell population required for tolerance mediated by various other agents. In Aim 2, we will define the roles of B effector cells, Th2 cells, and Treg in Breg induction and anti-TIM-1 mediated tolerance. In Aim 3 we will examine the mechanisms of Breg induction through TIM-1, identifying the role of cognate Ag recognition and specific cytokine signals. We believe these studies will greatly enhance our understanding of Breg immunobiology and provide therapeutic insight highly relevant to allograft tolerance.